Porous pipe has been used in underground irrigation systems. Underground irrigation using porous pipes have numerous advantages over above-ground watering. In above-ground watering, the water must enter the soil and penetrate through the root zone to benefit plants. With above-ground watering, water can be lost through evaporation and runoff. With underground irrigation systems, water is pumped through porous pipes and is delivered directly to the subsurface roots of the crop being cultivated. Optionally, nourishing fertilizer can be added to the feed water with minimum loss due to runoff. Additionally, air, herbicides, and/or insecticides can be directly distributed to the subsurface root zone on the soil using underground irrigation. Because the nutrients or other chemicals are directly distributed to the roots, instead of the soil surface, less chemicals are wasted and chemical requirements are minimized. This not only lowers costs significantly, but also is important to minimize potential damage to the environment. Porous flexible pipes of controlled and different porosity have also been used to diffuse fine bubbles of air and/or other gases into water and/or other liquids for use in effluent treating, bioremediation and fish farming.
Porous pipes used for underground irrigation processes usually are made from an extrusion process employing ground rubber crumbs and polyethylene particles as raw materials. U.S. Pat. Nos. 4,003,408; 4,110,420; and 4,168,799 issued to Turner disclosed porous irrigation pipes produced from reclaimed rubber mixed with polyethylene as a binder. The materials disclosed therein are incorporated herein by reference. The prescribed mixture is extruded by a wave screw. The extruder is equipped with a heating system to melt the thermoplastic polyethylene binder. During the heating and extruding process in the extruder, water vapor and any other gaseous components contained in the feed material expand to create porosity in the extrudate. The extruded pipe is cooled in a water bath to preserve its final form as a porous pipe. A venting capability is provided in the extruder to provide various degrees of vacuum and control the porosity of the end product porous pipe.
The porous pipes made according to the Turner process often suffer inconsistent quality problems, especially the variations in the porosity of the final product. U.S. Pat. Nos. 4,517,316; 4,615,642; and 4,616,055 issued to Mason disclosed methods which were attempts to improve the uniformity of porosity of the porous irrigation pipes. The contents of these disclosures are incorporated herein by reference. In these processes, the raw materials, which comprise reclaimed rubber tire treads and polyethylene particles, are first preprocessed into cylindrically shaped pellets form to provide a preformed feed material of desired composition and moisture content. Such a preprocessing step to form pellets might reduce the variability in the feed composition and reduce the surface area on which the moisture may be retained. Mason's patents also utilize additional constituents such as slip agents and lubricants to improve mixing inside the extruder. Another attempt to improve the uniformity of porosity of the flexible pipe of the porous pipes is disclosed in U.S. Pat. No. 4,958,770 issued to Mitchell. The content of the '770 patent is incorporated herein by reference. In the '770 patent, the crumb rubber and the polyethylene resin binder are thoroughly dried prior to their introduction to the extruder. The crumb rubber is fed into the extruder via a crammer screw, which delivers the dried crumb rubber into the extruder. Polyethylene resin is introduced into the crumb rubber via a polyethylene extruder before the crammer screw. The crammer screw actually delivers the crumb rubber with the accompanying polyethylene particles into the extruder. Since the feed materials have already been thoroughly dried, no venting is required in the extruder.
None of the prior art addresses the issue of feed segregation which often occurs in the path between the point in which the two feed materials are mixed, and the point right before the inlet of the extruder. Rubber crumbs and polyethylene particles have very different physical and rheological characteristics. They tend to segregate into either rubber-rich or polyethylene-rich populations before they enter the extruder. Such a feed segregation results in a porous pipe whose composition is nonuniform and different from the desired overall feed composition. As a consequence, the porosity as well as the strength of the porous pipe vary significantly along the length of the porous pipe made using these technologies.
Another issue that has been overlooked in the prior art processes is the varying degree of compaction exerted on the rubber crumbs before the feed enters the extruder. It is well known that rubber is a highly compactible material. Compaction can occur as a result of gravity head in the hopper or varying extruder pressure. Various degrees of gravity or different extruder pressures cause different degrees of compaction of the rubber crumbs and therefore result in varying concentrations of rubber crumbs in the feed. It has been disclosed in the prior art patents that the proportion of rubber crumbs significantly impacts the porosity of the final porous pipe product. Therefore, uncontrolled degree of compaction further causes uncontrolled porosity in the final product.
Due to the lack of control over the composition of the feed mixture in the extruder, truly automatic operation was not possible in any of the prior art processes. Automation of the manufacturing process generally requires the simultaneous adjustments of the post-extrusion tension applied on the final pipe, and the pressure and/or temperature inside the extruder barrel and at the diehead. A prerequisite of the automation process, however, is a controlled feed composition at a pre-determined value.